RNA polymerase organizes into distinct spatial clusters independent of ribosomal RNA transcription in E. coli

Recent studies have shown that RNA polymerase (RNAP) is spatially organized into distinct clusters in E. coli and B. subtilis cells. Spatially organized molecular components in prokaryotic systems imply compartmentalization without the use of membranes, which may offer new insights into pertinent functions and regulations. However, the function of RNAP clusters and whether its formation is driven by active ribosomal RNA (rRNA) transcription remain elusive. In this work, we investigated the spatial organization of RNAP in E. coli cells using quantitative superresolution imaging. We observed that RNAP formed large, distinct clusters under a rich medium growth condition and preferentially located in the center of the nucleoid. Two-color superresolution colocalization imaging showed that under the rich medium growth condition, nearly all RNAP clusters were active in synthesizing rRNA, suggesting that rRNA synthesis may be spatially separated from mRNA synthesis that most likely occurs at the nucleoid periphery. Surprisingly, a large fraction of RNAP clusters persisted under conditions in which rRNA synthesis was reduced or abolished, or when only one out of the seven rRNA operons (rrn) remained. Furthermore, when gyrase activity was inhibited, we observed a similar rRNA synthesis level, but multiple dispersed, smaller rRNA and RNAP clusters occupying not only the center but also the periphery of the nucleoid, comparable to an expanded nucleoid. These results suggested that RNAP was organized into active transcription centers for rRNA synthesis under the rich medium growth condition; their presence and spatial organization, however, were independent of rRNA synthesis activity under the conditions used but were instead influenced by the structure and characteristics of the underlying nucleoid. Our work opens the door for further investigations of the function and molecular nature of RNAP clusters and points to a potentially new mechanism of transcription regulation by the spatial organization of individual molecular components.